1. Field of the Invention
The present invention relates to a bulb mounting structure for a vehicular lamp for mounting a mounting piece of the bulb in a circular bulb insertion hole formed in a reflector of the lamp through a ring-shaped elastic seal member.
2. Related Art
FIG. 8 shows a conventional structure of the type described above. As shown in FIG. 8, a reflector 1 is mounted within a headlamp, and a circular bulb insertion hole 2 is formed through a central portion of the reflector 1. A bulb 4 having a mounting piece (hereinafter referred to merely as "bulb") is mounted in the bulb insertion hole 2 through an O-ring 3. The bulb 4 is constituted by a bulb body 5 and a mounting piece body 6 formed of a synthetic resin for supporting the bulb body 5. The mounting piece body 6 includes a disk-shaped engagement portion 7 engageable with the bulb insertion hole 2, and a concave groove 7a is formed in an outer peripheral edge or surface of the disk-shaped engagement portion 7, and the O-ring 3 is received in the concave groove 7a.
A bayonet portion 8 is formed on and projects from the mounting piece body 6 whereas an engagement groove 9 for retainingly receiving the bayonet portion 8 is formed in an inner peripheral surface of the bulb insertion hole 2. The disk-shaped engagement portion 7 is inserted into the bulb insertion hole 2 and angularly moved or turned to engage the bayonet portion 8 in the engagement groove 9, thereby fixing the bulb 4 to the reflector 1.
When the bulb 4 is thus fixed relative to the bulb insertion hole 2, the O-ring 3 in the concave groove 7a is pressed against the inner peripheral surface of the bulb insertion hole 2, thereby sealingly closing a gap between the bulb insertion hole 2 and the bulb 4.
According to the above conventional structure, the width W of the concave groove 7a is substantially equal to the diameter D of a cross-section of the O-ring 3, as shown in FIG. 9, and therefore the contact pressure P applied to the compressed O-ring 3 against the inner peripheral surface of the bulb insertion hole 2 is so excessive that the bulb 4 is not smoothly attached and detached relative to the bulb insertion hole 2.
Furthermore, when the bulb 4 is inserted into the bulb insertion hole 2, a part of the O-ring 3 may protrude beyond an edge 7b of the concave groove 7a in a direction opposite to the direction of insertion of the bulb 4 as shown in FIG. 9, and the O-ring 3 may be lifted out of a bottom surface of the groove 7b as shown in FIG. 10. When the bulb 4 is turned to engage the bayonet 4 with the engagement groove 9, the protruding or the lifting of the O-ring 3 is promoted, so that the contact pressure applied by the O-ring 3 becomes uneven in the circumferential direction, thus causing the sealing effect to be unstable. Moreover, it is possible that a crack develops in the protruding portion of the O-ring 3 pressed against the edge 7b of the groove 7, and it is also possible that a gap is formed at the area of contact of the O-ring 3 because of the above lifting. As a result, the sealing of the bulb insertion hole 2 may be inadequate.